(a) Fields of the Invention
The present invention relates to solid-state image sensing devices having a solid-state image sensing element and a transparent substrate for protecting the element, and to their fabrication methods.
(b) Description of Related Art
In a solid-state image sensing device employing a CCD (Charge Coupled Device) and the like, the area of a photodiode serving as a light receiving unit has been decreasing by demands for downsizing and resolution enhancement thereof. Such a decrease in the area of the light receiving unit in turn degrades the light collection efficiency of the device. In order to make up for this degradation, a so-called microlens has come to be developed and then put into use. This microlens is typically made of resin and disposed above the light receiving unit formed on each pixel. The microlens refracts light not coming directly in the light receiving unit to collect the refracted light into the light receiving unit, thereby enhancing light collection efficiency to improve sensitivity.
FIGS. 15 and 16 show cross-sectional structures of a conventional solid-state image sensing device. Referring to FIG. 15, in the surface of a substrate 101 for a CCD-type solid-state image sensing element, a recess is provided on each pixel. At the bottom of the recess, a photodiode 102 is provided which converts an incoming light into an electrical signal. On the substrate 101 for the solid-state image sensing element, a first acrylic flattening film 103 is formed which flattens unevenness of the substrate surface. On the first acrylic flattening film 103, color filters 104 are formed to be associated with the photodiodes 102, respectively. On the color filters 104, a second acrylic flattening film 105 is formed which flattens unevenness generated due to gaps between the color filters 104. On the second acrylic flattening film 105, microlenses 106 are formed to be associated with the photodiodes 102, respectively.
Referring to FIG. 16, a solid-state image sensing element 113 is composed of the photodiodes 102, the color filters 104, the microlenses 106, and the like formed on the substrate 101 for the solid-state image sensing element. The solid-state image sensing element 113 is mounted inside a package 112, and the top of the package 112 is covered with a transparent substrate 109. As shown in FIG. 15, inside the package 112, an air space 110 is interposed between the solid-state image sensing element 113 and the back surface of the transparent substrate 109. As shown in FIG. 15, when light 111 transmits through the transparent substrate 109 to come in the microlenses 106, reflections occur on the top and bottom surfaces of the transparent substrate 109 and the top surfaces of the microlenses 106.
However, rapid downsizing of the device in recent years has made it difficult to secure sufficient sensitivity only by light collection from the microlenses. To overcome such a difficulty, a structure such that an antireflection film is formed on the microlenses is proposed (see Patent Document 1: Japanese Patent No. 2719238).    (Patent Document 2)            Japanese Examined Patent Publication No. 7-54974            (Patent Document 3)            Japanese Examined Patent Publication No. 7-28014            (Patent Document 4)            Japanese Patent No. 2942369        